Study of water uptake mechanisms in plants living in gypsum
Gypsum has water in its crystalline structure (CASO4 • 2H2O) and depending on the temperature, pressure or presence of solutes this water can be naturally lost. Crystallization water represents 20.8% of the total weight of gypsum and previous studies show that it constitutes a source of water for organisms living in this substrate, especially under drought conditions. However, the mechanisms displayed by plants to gain access to crystalline water remain unexplored. Similarly, the ecological implications that this new source of water can have for the structure of the communities and their ecophysiology is unknown.
Here we present a set of experiments being currently undertaken to shed light on these issues.
• The gypsophite Helianthemum squamatum (Fig 1) is being cultivated in pots with natural gypsum soil and soil where the crystalline water has been labeled with deuterium. Half of the pots of each soil type have been sterilized with γradiation
and subsequently watered with a filtrate incorporating a natural soil bacteria inoculum, so as to produce fungusfree soil. We plan to overimpose a drought treatment to half of these plants and measure the differences in water uptake and in gypsum crystalline water use. This will inform on the role of fungi for crystalline water uptake.
• H. squamatum and its sister gypsovag species Helianthemum syriacum (Fig 2) are being cultivated in rhizotrons with fungus free soil and natural soil (following γsterilization and addition of a bacteria filtrate) where changes in the pH of the rhizosphere will be monitored (Fig 3). This will indicate the ability of these plants to modify gypsum soil pH.
• We have designed minirhizotrons (Fig 4) compatible with crystallographic measurements and are cultivating H. squamatum in them. Plants will be subjected to drought and different analyses (Raman spectrometry, gasexchange, XRD, tomography) will b e c arried out a t the p lant and rootsoil
interface to detect changes in the thermodynamic phases of gypsum and gypsum dissolution processes mediated by root activity.
• Finally, to detect differences in the use of crystalline water among plants growing in the same community, we carried out a field survey of the xylem sap of 20 dominant species of the gypsum community in Alfajarín, Zaragoza both in spring and summer, and characterized the isotopic composition water table and soil water along three 1 m depth soil profiles.